“Daddy! Amy’s baby was Flesh, but it’s all right cos Melody was River Song as a grown-up!” – shrieked Rosie, running to the next room to tell my brother-in-law, who was more interested in sport on the radio. My bright little niece, aged three, was totally clued up on all the week’s surprises. And ready to explain them to any nonplussed adult.

I love watching Doctor Who with little ones: it reminds me of the magnetic hold the programme had over me at their age – that irresistible mix of fascination and terror. They may not grasp every nuance on first viewing, but children make the necessary leaps of faith.

“This little girl. It’s all about her!” the Doctor reminds us at the top of the episode. It was ever thus with Steven Moffat. As well as the girl in the astronaut suit (Melody/River), he’s given us a girl in the fireplace (Reinette), a girl in the library (Charlotte), the little girl who waited (Amelia Pond)… Even Blink’s Sally Sparrow was a child in Moffat’s original short story. And it all kicked off memorably with a boy in a gas mask.

Now we’re onto babies. I’m under strict instructions from Rosie’s mum not to make any glib remarks about Amy’s dissolving sprog. As if I would! (Banana blancmange…) But it is a horrible image guaranteed to disturb every parent.

Although I guessed River would be Rory and Amy’s baby a while back, it’s still a pleasure to watch her divulge the information after three years of secrecy. The four leads must have been cock-a-hoop to turn these script pages for the first time, and their reactions on screen are expertly nuanced. Alex Kingston manages an extraordinary combination of teasing/relieved/happy/sad, while Arthur Darvill does something miraculous with his entire scalp in the final shot.

So now we’ve witnessed River’s death and birth, her last and first meetings with the Doctor, and many points between. I defy you to rewatch all her episodes and construct a coherent timeline.

Looking back at last year’s Flesh and Stone, we can see River was saving her own mum Amy’s life before “Melody” was even conceived. In The Big Bang, River passed by her parents’ wedding like an apparition; that was the night she was conceived, which adds a double meaning to the episode title. And now perhaps we know why she muttered, “Of course not” when she failed to shoot The Impossible Astronaut.

A Good Man Goes to War confounds expectations. You might have supposed from a photo published in RT (16 April) that the Cybermen would dominate this episode. In fact they appear only in the lustrous pre-titles tease, almost panicking over an intruder at their space station, who turns out to be a mightily emboldened Rory. (Great to see the Cyberfleet retaining designs from The Invasion, 1968.)

A Sontaran becomes a nurse, and I love the idea of a lady Silurian leading a covert life as a Victorian avenger, polishing off Jack the Ripper. Vastra and her devoted maid/lover Jenny deserve a spin-off series. This lizard/lesbian undercurrent (homo reptilia, anyone?) proves that the perceived “gay agenda” of the Russell T Davies era is not dead. As do the self-declared “thin/fat gay married Anglican marines” and screamingly camp Dorium Maldovar – “I’m old. I’m fat. I’m blue!”

I haven’t done a head count but Moffat must be emulating Seven Samurai/The Magnificent Seven with this motley crew of ne’er-do-wells turned goodies.

Loads of characters turn up for a sort of “Hey presto!” flourish: Avery and son, now free of life-support tubing (RT removed their names from the cast list to preserve the surprise); “Danny Boy” and the Spitfires from Victory of the Daleks now travelling in time; and a Silurian army no longer hibernating on Earth but able to materialise on the spot. No time for explanations. It’s just happened and should be accepted as a bit of cartoonish fun.

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In all, a rather fabulous episode in which Moffat again gets to pull out the big guns. I envisage the other writers must envy him while they get lumbered with limited casts and budget. They’re like kids of yore making do with hand-me-down Lego while Steven is the little boy with the bulging, flashy toy box.